A typical cancer cell has thousands of mutations scattered throughout its genome and hundreds of mutated genes. However, only a handful of those genes, known as drivers, are responsible for cancerous traits such as uncontrolled growth. Cancer biologists have largely ignored the other mutations, believing they had little or no impact on cancer progression. But a new study reveals, for the first time, that these so-called passenger mutations are not just along for the ride. When enough of them accumulate, they can slow or even halt tumor growth.